We have demonstrated high-precision cutting and drilling of CFRP using a 3-kW CW single-mode fiber laser and a galvanometer scanner. A 3-mm-thick thermoset CFRP was successfully cut with 100 scans at the scanning speed of 13 m/s, which corresponds to effective cutting speed of 7.8 m/min. We employed the multi-pass method, in which a laser is scanned on the same position at high speed, and the optimized scanning position shift to realize short-time cutting. The time interval between scans was less than 20 ms, which was much shorter than general time interval in multi-pass method. Even with such short interval, the width of heat-affected zone (HAZ) was controlled to 97 μm on average. Drilling demonstration was carried out by using the same setup as the cutting experiment. Holes with a diameter of 6.4 mm have been successfully drilled on a 2-mm-thick thermoset CFRP. The processing time was 2.7 seconds, which was equivalent to that in mechanical processing. Since a HAZ tends to expand in the direction of the carbon fibers due to their high heat conductivity, it is difficult to reduce the width of HAZ in all directions around the processed area. To overcome this challenge, we optimized the gas injection condition as well as the scanning condition. As a result, the widths of HAZs were successfully controlled to about 100 μm in all directions.
A high power single-mode fiber laser has received a lot of attention in various materials processing fields. In order to carry out processing under proper conditions with single-mode fiber laser, the dynamic flows of fluids inside the materials should be precisely understood. We carried out the numerical analysis of materials processing using a 5-kW single-mode fiber laser. The dynamic flows of the copper fluid during the bead-on-plate tests are calculated and the results agree well with the experimental results. The processing with the different scan speed and spot size, and at the output power of 10 kW are numerically calculated as well. The proper processing conditions with less spatters and dross for each materials can be determined by the dynamic analysis.
Stimulated Raman scattering gives back reflection sensitivity to a high power fiber laser. Therefor SRS suppression is necessary in order to realize stable laser processing by a high power fiber laser. A 5-kW single-mode ytterbium doped fiber laser with a 20-m long delivery fiber has been realized. The fiber laser is an all-fiber single-stage Fabry-Perot system in a co-pumping configuration. The optical to optical efficiency was 80% at the output power of 5.0 kW. And the M-squared figure of 1.3 was obtained. The Stokes light by SRS is suppressed to 45 dB below the laser output by using fibers with the effective mode area of 600 μm2 . While SRS was well suppressed, four wave mixing was observed with the frequency shift of ~6 THz. Four wave mixing between the fundamental mode and the secondary modes is believed to take place. Four wave mixing is believed not to give back reflection sensitivity to the fiber laser. The 5-kW single-mode fiber laser was applied to laser processing. Bead-on-plate tests were carried out with a galvanometer scanner. The laser ran without stopping nor damaging the laser system even during processing highly reflective material. This implies that our SRS suppressed single-mode fiber laser can be used practically in most of processing systems.
A 3 kW single stage all-fiber Yb-doped single-mode fiber laser with bi-directional pumping configuration has been demonstrated. Our newly developed high-power LD modules are employed for a high available pump power of 4.9 kW. The length of the delivery fiber is 20 m which is long enough to be used in most of laser processing machines. An output power of 3 kW was achieved at a pump power of 4.23 kW. The slope efficiency was 70%. SRS was able to be suppressed at the same output power by increasing ratio of backward pump power. The SRS level was improved by 5dB when 57% backward pump ratio was adopted compared with the case of 50%. SRS was 35dB below the laser power at the output power of 3 kW even with a 20-m delivery fiber. The M-squared factor was 1.3. Single-mode beam quality was obtained. To evaluate practical utility of the 3 kW single-mode fiber laser, a Bead-on-Plate (BoP) test onto a pure copper plate was executed. The BoP test onto a copper plate was made without stopping or damaging the laser system. That indicates our high power single-mode fiber lasers can be used practically in processing of materials with high reflectivity and high thermal conductivity.
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